derivations: Raise an error for references to non-existent outputs.

Fixes <http://bugs.gnu.org/19630>.
Reported by Ricardo Wurmus <ricardo.wurmus@mdc-berlin.de>.

* guix/derivations.scm (&derivation-error,
  &derivation-missing-output-error): New error conditions.
  (derivation->output-path): Raise a '&derivation-missing-output-error'
  if OUTPUT is not an output of DRV.
* guix/ui.scm (call-with-error-handling): Add case for
  'derivation-missing-output-error?'.
  (show-what-to-build): Check whether (derivation-outputs drv) is
  empty.
* tests/packages.scm ("reference to non-existent output"): Add test.

guix/derivations.scm

@@ -21,6 +21,8 @@ (define-module (guix derivations)
   #:use-module (srfi srfi-9)
   #:use-module (srfi srfi-9 gnu)
   #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-34)
+  #:use-module (srfi srfi-35)
   #:use-module (rnrs io ports)
   #:use-module (rnrs bytevectors)
   #:use-module (ice-9 match)
@@ -59,6 +61,13 @@ (define-module (guix derivations)
             derivation-input-sub-derivations
             derivation-input-output-paths
 
+            &derivation-error
+            derivation-error?
+            derivation-error-derivation
+            &derivation-missing-output-error
+            derivation-missing-output-error?
+            derivation-missing-output
+
             derivation-name
             derivation-output-names
             fixed-output-derivation?
@@ -97,6 +106,18 @@ (define-module (guix derivations)
 
   #:replace (build-derivations))
 
+;;;
+;;; Error conditions.
+;;;
+
+(define-condition-type &derivation-error &nix-error
+  derivation-error?
+  (derivation derivation-error-derivation))
+
+(define-condition-type &derivation-missing-output-error &derivation-error
+  derivation-missing-output-error?
+  (output derivation-missing-output))
+
 ;;;
 ;;; Nix derivations, as implemented in Nix's `derivations.cc'.
 ;;;
@@ -509,9 +530,15 @@ (define derivation->string
         (cut write-derivation drv <>))))))
 
 (define* (derivation->output-path drv #:optional (output "out"))
-  "Return the store path of its output OUTPUT."
-  (let ((outputs (derivation-outputs drv)))
-    (and=> (assoc-ref outputs output) derivation-output-path)))
+  "Return the store path of its output OUTPUT.  Raise a
+'&derivation-missing-output-error' condition if OUTPUT is not an output of
+DRV."
+  (let ((output* (assoc-ref (derivation-outputs drv) output)))
+    (if output*
+        (derivation-output-path output*)
+        (raise (condition (&derivation-missing-output-error
+                           (derivation drv)
+                           (output output)))))))
 
 (define (derivation->output-paths drv)
   "Return the list of name/path pairs of the outputs of DRV."

guix/ui.scm

@@ -249,6 +249,10 @@ (define (call-with-error-handling thunk)
              ;; FIXME: Server-provided error messages aren't i18n'd.
              (leave (_ "build failed: ~a~%")
                     (nix-protocol-error-message c)))
+            ((derivation-missing-output-error? c)
+             (leave (_ "reference to invalid output '~a' of derivation '~a'~%")
+                    (derivation-missing-output c)
+                    (derivation-file-name (derivation-error-derivation c))))
             ((message-condition? c)
              ;; Normally '&message' error conditions have an i18n'd message.
              (leave (_ "~a~%")
@@ -309,9 +313,8 @@ (define substitutable?
         (const #f)))
 
   (define (built-or-substitutable? drv)
-    (let ((out (derivation->output-path drv)))
-      ;; If DRV has zero outputs, OUT is #f.
-      (or (not out)
+    (or (null? (derivation-outputs drv))
+        (let ((out (derivation->output-path drv))) ;XXX: assume "out" exists
           (or (valid-path? store out)
               (substitutable? out)))))
 

tests/packages.scm

@@ -268,6 +268,17 @@ (define read-at
       (package-derivation %store p)
       #f)))
 
+(test-assert "reference to non-existent output"
+  ;; See <http://bugs.gnu.org/19630>.
+  (let* ((dep (dummy-package "dep"))
+         (p   (dummy-package "p"
+                (inputs `(("dep" ,dep "non-existent"))))))
+    (guard (c ((derivation-missing-output-error? c)
+               (and (string=? (derivation-missing-output c) "non-existent")
+                    (equal? (package-derivation %store dep)
+                            (derivation-error-derivation c)))))
+      (package-derivation %store p))))
+
 (test-assert "trivial"
   (let* ((p (package (inherit (dummy-package "trivial"))
               (build-system trivial-build-system)